The invention relates to a method for the continuous production of a coarse-grained ammonium sulphate product by crystallization of an ammonium sulphate solution in a crystallization stage, which is operated in accordance with the draft tube baffled (DTB) principle and to an installation for carrying out this method.
Ammonium sulphate ((NH4)2SO4) is a product which is produced on a large scale and is used mainly as fertilizer in agriculture to provide nitrogen and sulphur. In industrial terms, ammonium sulphate is produced as a by-product in some chemical processes, in particular in the production of caprolactam. In order to satisfy the requirements as a fertilizer, the ammonium sulphate should be available as a coarse-grained product (grain size diameter (RRSB) in the range of 2 to 3 mm). This is important in order to ensure effective scattering power and, when being mixed with other fertilizing substances, to suppress the tendency towards demixing which would be increased by fine-grain product in a grain mixture.
In order to produce the coarse-grained crystallizate from an ammonium sulphate solution, crystallizers of the DTB type are very frequently used. This gives rise to the problem that the average grain size, produced in the crystallizer, of the crystallizate which is drawn off in a suspension is subjected to periodic fluctuations, i.e., phases with a high coarse grain proportion alternate with phases, in which predominantly fine-grained crystallizate (e.g., grain size of less than 1.5 mm) is produced. The reasons for this are set out hereinafter.
During the course of the operating period, the size of the formed crystals initially increases constantly, since by reason of the fine-grain discharge by the outer solution circuit of the crystallizer only the crystals which become ever larger are available for breaking down the oversaturation. The rate of oversaturation break-down on the crystals is, in a first approximation, proportional to the product of diffusion rate and available crystal surface area. Since the specific surface area of the crystal stock in the crystallizer becomes smaller as the grain becomes ever larger, the oversaturation must become continuously larger in order to achieve the production output specified by evaporation. If the oversaturation then exceeds the metastable range, spontaneous nucleation occurs suddenly with a myriad of very fine crystals. Although they are dissolved for the most part in the external circuit, a proportion of the fine crystals still remains in the internal circulation circuit and grows to the grain size which can no longer be discharged via the clarifying zone, but initially is still considerably smaller than the grain size intended for the product. Over time, these crystals then grow to the desired grain size. After a further increase in the grain size, spontaneous nucleation then occurs again, i.e., the cycle starts anew.
EP 0632738 B1, which is a generic type, discloses a continuous crystallization method, in which a coarse-grained ammonium sulphate crystallizate can be produced from an oversaturated, aqueous ammonium sulphate solution in a DTB crystallizer. In this method, a suspension of oversaturated ammonium sulphate solution and already formed crystallizate is constantly circulated within the DTB crystallizer in an internal circuit. By means of evaporation of the solvent (water), new oversaturation is continuously produced which is then broken down as a consequence of the crystallization which occurs. The vapour produced during evaporation is drawn off at the head of the DTB crystallizer. From a part which is separated from the internal circuit of the suspension by means of flow guide walls in the upper region of the crystallization chamber of the DTB crystallizer and in which, in contrast to the base region of the crystallizer, a clarified solution is present having a solid proportion consisting substantially of crystallization nuclei and fine crystals, a partial flow of clarified solution is drawn off and, after dissolution of the solid proportion contained therein, is then guided back into the base region of the crystallization chamber. In order to dissolve the solid proportion, a heat exchanger is connected into the external circuit and elevates the temperature of the clarified solution and thus the power of the solvent to dissolve ammonium sulphate. Moreover, the supply line, through which new concentrated ammonium sulphate solution can be fed into the crystallizer, also issues into the external circuit upstream of the heat exchanger. A suspension flow with the proportion of solids contained therein at the desired grain size of the product crystallizate is continuously drawn off from the base region. The product crystallizate is separated from the mother liquor or solution in a thickener and by subsequent centrifugation and the mother solution is then guided back into the DTB crystallizer.
In order to increase production of a sufficiently coarse-grained crystallizate and to improve production with regard to cyclical fluctuations in grain size, in this method a crystallizate suspension of ammonium sulphate is fed at a constant inflow rate from an external source into the crystallizer, in addition to the supply of saturated ammonium sulphate solution. EP 0632738 B1 does not contain any references to the way in which the suspension is produced, i.e., whether it has been produced for instance by comminution of product crystallizate or in a separate crystallizer. It is merely specified that this suspension must satisfy specific conditions: the temperature of the suspension fed in must not exceed the operating temperature in the crystallizer. Moreover, the suspension must contain 6-24 vol. % crystallizate, wherein at least 35% of the crystals are larger than 1.2 mm, and the feed of the suspension is to be dimensioned such that the weight of the crystals in the suspension fed in is in the range of 4-25% of the weight of the crystals in the suspension with the product crystallizate, said suspension being drawn off from the base region of the crystallizer.
A controlled feed of crystal suspension into a crystallizer to influence the grain size is also defined as seeding.
WO 00/56416 discloses a method for controlling the grain size in continuous mass crystallization which is also provided for producing coarse-grained ammonium sulphate crystallizate in a DTB crystallizer and in which, in a similar manner to the method in accordance with EP 0632738 B1, seeding is effected with an externally supplied crystal suspension. The seeding product is a crystallizate which is produced in its parameters independently of the current crystallization process and which has an average grain diameter of 0.1-1.0 mm. In this method, the temperature of the seeding product during the addition also must not be higher than the operating temperature in the crystallizer, but must be up to 40° C., preferably 10-30° C., less. All other feeds and recirculations are free of solids. In particular, this means that the feedstock for producing the ammonium sulphate product crystallizate is supplied as a preheated, solid-free ammonium sulphate feed solution, and that the external circuit consisting of the drawn-off solution with a fine solid proportion leads from the crystallizer initially to a heat exchanger which, by elevating the temperature of the solution, causes the solid proportion to re-dissolve, before the solid-free solution is then guided back into the crystallizer. The heat energy for heating the heat exchanger is provided by means of the vapour which is drawn off from the crystallizer and is brought to a higher temperature level initially by means of vapour compression. From the base region of the crystallizer, a suspension with a solid proportion at the desired grain size is drawn off in a continuous manner and is separated by centrifugation into product crystallizate and mother solution, wherein the mother solution is conveyed into an intermediate vessel from where it is fed back into the circulating line of the external circuit of the crystallizer. The seeding product is preferably added in a quantity, the solid proportion of which amounts to 5-30 wt. % of the solid discharged from the crystallizer in each case. The solid proportion of the seeding product can be produced e.g., by mechanical comminution of a portion of the product crystallizate and/or by a separate crystallization stage.
JP2005-194153 A discloses an installation, which is designed as a DTB crystallizer, for the production of ammonium sulphate crystallizate, in which there is provided an external circuit for clarified solution which is connected to a clarifying zone of the crystallizer and into which optionally there is incorporated a heat exchanger or a supply line for solvents (e.g., water or undersaturated ammonium sulphate solution) for the purpose of dissolving the contained solid proportion. Moreover, the clarifying zone is connected directly or indirectly via the external circuit to a further discharge line, by means of which a suspension which contains only fine crystallizate as the solid can be discharged completely from the process, in order to obtain, e.g., fine-grained ammonium sulphate product. Furthermore, connected to the uppermost part of the clarifying zone is also a third discharge line, by means of which any excess crystallization nuclei and superfine crystallizate can be drawn off and guided into a collecting vessel where required. In order to dissolve the solids, solvent is added to the collecting vessel from where the solution obtained is guided into a neutralisation vessel, in which it is added to sulphuric acid and ammonia and is heated by the neutralization reaction associated therewith. The heated, solid-free solution is then fed into the crystallizer.
In order to avoid the considerable grain size fluctuations in the ammonium sulphate crystallization, DE 102008007154 A1 provides a two-stage crystallization method with DTB crystallizers, in the first stage of which merely a fine crystal suspension is produced which is used for feeding the second stage, in which the coarse crystallizate is produced. In this case, a sufficient quantity of fine crystals to achieve the desired crystal growth is always provided for the second crystallization stage.
In order to avoid periodic fine grain formation in a DTB crystallizer, DE 102008029050 A1 proposes a seeding method, in which a fine crystal suspension which is produced preferably by means of flash-crystallization is fed continuously into the internal suspension circuit of the crystallizer, wherein the quantity of seeding suspension fed in is regulated by observing the fine crystallizate proportion in the internal suspension circuit.